This invention relates to valve timing-changeover control for an internal combustion engine which is capable of changing the valve timing of inlet and exhaust valves, and particularly to a method of effecting failsafe operation in connection with the valve timing control when there arises abnormality in the engine control system, e.g. when a sensor for detecting operating conditions of the engine is faulty.
An engine is conventionally known, which is capable of changing the valve timing of inlet valves and/or exhaust valves between low speed valve timing suitable for a lower engine rotational speed region and high speed valve timing suitable for a higher engine rotational speed region, e.g. from Japanese Patent Publication (Kokoku) No. 49-33289. Amongst methods of controlling changeover of the valve timing and fuel injection associated therewith, the following two methods have been proposed:
(1) A method, proposed by Japanese Provisional Utility Model Publication (Kokai) No. 61-157143, which comprises:
setting a high speed valve timing region in which high speed valve timing is to be selected and a low speed valve timing region in which low speed valve timing is to be selected, in accordance with the engine rotational speed Ne and the intake pipe absolute pressure P.sub.BA, and changing the valve timing between the high speed valve timing and the low speed valve timing in response to Ne and P.sub.BA ; and
at the same time setting a basic fuel injection period for determining the amount of fuel to be supplied to the engine by the use of a map of the engine rotational speed Ne and the intake pipe absolute pressure P.sub.BA, in which map values of the basic fuel injection period are so set as to obtain air-fuel ratios optimum for the high speed valve timing and low speed valve timing selected.
(2) A method, proposed by Japanese Patent Application No. 63-192239 filed by the assignee of the preset application, which comprises:
providing a basic fuel injection period map set for low speed valve timing and a basic fuel injection period map set for high speed valve timing;
changing the valve timing when two values respectively read from the two maps depending on engine operating condition are substantially equal to each other; and
determining the amount of fuel to be supplied to the engine based on a value of basic fuel injection period read from the high speed valve timing map when the high speed valve timing is selected, and based on a value of basic fuel injection period read from the low speed valve timing map when the low speed valve timing is selected.
In an engine and a control system therefor to which the above conventional method (1) or (2) is applied, the changeover of the valve timing is specifically carried out in the following manner:
An instruction signal is supplied from an electronic control unit to an electromagnetic valve to open and close same, which in turn causes an oil pressure-changeover valve to be opened and closed. This changes the oil pressure supplied to a valve timing-changeover mechanism which operates in response to the resulting high or low oil pressure to change the valve timing between high speed valve timing and low speed valve timing.
In connection with the above-described valve timing control, a failsafe method has been proposed by the assignee of the present application in the above-mentioned Japanese Patent Application No. 63-192239, in which when abnormality in an engine operating parameter sensor or the like is detected, the valve timing is held at the low speed valve timing irrespective of engine operating conditions. However, this method suffers from the following problems:
When abnormality in an engine operating parameter sensor or the like is detected, an instruction signal for changing the valve timing to the low speed valve timing is supplied from the electronic control unit to the electromagnetic valve. However, there may be a case in which due to failure of the oil pressure-changeover valve or the valve timing-changeover mechanism, the valve timing is not actually changed to the low speed valve timing and continually held at the high speed valve timing.
In this case, it can happen that even when the engine operating condition is in the low speed valve timing region, the valve timing is actually held at the high speed valve timing. If the above failsafe method is applied to the first-mentioned method (1) in such case, it gives rise to the following inconveniences: For example, if the low speed valve timing region and the high speed valve timing region are set as shown in FIG. 14, when the engine is in a fairly high speed region above 3,000 rpm (even though the engine rotational speed is lower than a predetermined value Ne1 at which valve timing changeover is to be made), the amount of fuel to be supplied to the engine is determined based on the basic fuel injection period for the low speed valve timing in spite of the fact that the actual valve timing is held at the high speed valve timing as described above, so that the air-fuel ratio of the mixture becomes lean, resulting in increased combustion temperature or increased temperature of a catalyst in the exhaust gas purifying means (e.g. above 1,000.degree. C.). This may cause preignition, which leads to melting of ignition plugs and knocking at a high engine rotational speed, shortened life of the catalyst, etc.
On the other hand, if the above failsafe method is applied to the second-mentioned method (2), when abnormality in an engine operating parameter sensor or the like is detected, the electronic control unit generates an instruction signal for changing the valve timing to the low speed valve timing, and the amount of fuel to be supplied to the engine is determined based on the basic fuel injection period read from the low speed valve timing map. However, it can happen that although the valve timing is actually held at the high speed valve timing, the amount of fuel to be supplied to the engine is determined based on fuel injection period read from the low speed valve timing map.
Since values of the basic fuel injection period for the low speed valve timing are set at greater values than those for the high speed valve timing in the lower engine speed region, and inversely smaller than those for the high speed valve timing in the higher engine speed region, if in the higher engine speed region, the fuel amount is determined based on the basic fuel injection period read from the low speed valve timing map in spite of the fact that the valve timing is actually held at the high speed valve timing, the air-fuel ratio of the mixture becomes lean, which gives rise to the same inconveniences as stated above concerning the method (1).
The above-mentioned inconveniences are conspicuous particularly when the engine is in an operating region in which the engine rotational is high and at the same time load on the engine is high.